Goodfellow and Orchard investigated antibiotic sensitivity patterns of 151 nocardioform bacterial strains representing Nocardia, Gordona, 'Mycobacterium' rhodochrous, and bagasse isolates using filter-paper disc methodology. Strains were tested against 52 antimicrobial agents at various concentrations on Diagnostic Sensitivity Test agar. Results showed increasing antibiotic sensitivity across species, with bagasse isolates being most resistant and nocardiae showing moderate sensitivity. Notably, nocardiae were effectively inhibited by low concentrations of erythromycin, miconazole, gentamicin, and tobramycin. The bagasse isolates showed resistance to most agents but susceptibility to high concentrations of certain cephalosporins, tetracyclines, and benzyl-penicillin. Aminoglycosides proved most active against nocardiae and rhodochrous strains. The authors demonstrated that antibiotic sensitivity patterns were reproducible and provided taxonomically useful characters for differentiating nocardioform taxa, particularly supporting the classification of Nocardia sensu stricto. These findings suggest antibiotic sensitivity testing could improve both taxonomic classification and clinical treatment guidance for Nocardia infections.

Key findings

• Antibiotic sensitivity patterns showed increasing susceptibility from bagasse isolates, to gordonae, to nocardiae, to rhodochrous strains, forming a consistent series useful for taxonomy
• Nocardiae were most effectively inhibited by aminoglycosides (gentamicin and tobramycin), macrolides (erythromycin), and certain tetracyclines (doxycycline and minocycline)
• Antibiotic sensitivity patterns were reproducible and provided reliable characters for differentiating and identifying nocardioform bacteria taxa
• Specific antibiotic concentrations could distinguish Nocardia sensu stricto from related genera Gordona and 'Mycobacterium' rhodochrous
• Miconazole inhibited all strains at low concentrations despite its primarily antifungal classification, suggesting broader antimicrobial applications